I had a used Echo 1 P90 that I bought for $100; was totally awesome and performed more like a rifle than a compact PDW-type weapon. Fire rate was nice with a 9.6v battery, and it was accurate out to 100+ feet. For longevity, however, I have no idea, as I only had it for about a month.
Although, if you get one, DO NOT buy the high-capacity magazines, they are the junkiest things ever (they don't feed properly and can't be wound while they're in the gun). I used some MAG mid-caps and had no problems.